1. Field of the Invention
The present invention relates to a collapsible mold and to a method of manufacturing the same.
2. Description of Related Art
For high pressure die casting, many techniques for casting a product having an undercut have been heretofore devised in which a collapsible core mainly containing sand or salt is used to cast the product, and this then is dissolved and removed after the casting. For the purpose of improving the collapsibility of the core, a method is known in which an alkaline-earth metal compound that swells when reacting with water is added to a material for a core mainly containing a water-soluble component such as a salt (refer to JP 2006-7234 A, for example). However, the core manufactured by mixing hydrated lime (calcium hydroxide), calcined lime (calcium oxide) or the like as the alkaline-earth metal compound needs to be stored in a humidity controlled container. This is because an alkaline-earth metal oxide generated from thermal decomposition of the alkaline-earth metal compound absorbs moisture in the atmosphere and thus swells after molding. In addition, even if the core is stored in a low humidity environment, the core inevitably absorbs moisture and swells. For this reason, the core cannot be stored for a long time. Furthermore, the core, having once absorbed moisture and swelled, has a problem of having such a rough surface that a good casting surface cannot be obtained at the time of casting. Moreover, such a core also has other problems of having decreased strength, and of causing a porosity in a casting due to the re-discharge of moisture at the time of the casting.
Furthermore, investment casting methods including ceramic shell molding and solid molding are used for a precision casting or a glass casting (hereinafter, referred to as a lost-wax mold). One of the methods of manufacturing a mold for investment casting that has been developed involves: molding, by use of a wax copy of an original model (hereinafter, referred to as a wax pattern), a mold composition containing calcium carbonate as a refractory; melting in the mold and draining out the wax pattern (dewaxing); and burning the resultant mold composition (green mold) at a temperature not less than the decomposition temperature of calcium carbonate. When the mold manufactured by this method is used, calcium oxide contained in the mold can be slaked and then turned into calcium hydroxide by immersing the mold in water or leaving the mold to stand in the air after the casting. At this time, the mold self-collapses due to an increase in the volume of the mold, thus making it easier to remove the mold from the casting.
In addition, a method of manufacturing a mold having good air permeability has been devised, focusing on the point where calcium carbonate becomes porous by the thermal decomposition at the time of burning. For example, there is a method of manufacturing a lost-wax mold, involving: preparing a mold containing not less than 10 wt % of calcium carbonate; and after a dewaxing process, turning a part of the calcium carbonate into calcium oxide by burning the mold at a temperature not less than 850° C. (refer to JP 49-2655 B, for example). In addition, a method of manufacturing a lost-wax mold by using fossil shell as a source of calcium carbonate is known (refer to JP 6-36954 B, for example). Since fossil shell is used as the calcium carbonate source, a mold having good collapsibility can be obtained even when the mold is burned at a burning temperature as low as 760° C.
Furthermore, a manufacturing method using a mixture of calcium carbonate or calcium carbonate and a refractory as a filler and using colloidal silica as a binder is known (refer to JP 3-281030 A, for example). According to this method, gelation of a slurry made of the mixture of the calcium carbonate or the calcium carbonate and the refractory is prevented for a long period of time. This facilitates molding of a mold, and at the same time, provides collapsibility after casting. Furthermore, there are known a manufacturing method using a stucco material mixed with 10 to 80 wt % of calcium carbonate (refer to JP 5-104199 A, for example), and a method of manufacturing a mold with good collapsibility by using calcium carbonate in a part of a refractory layer (refer to JP 6-15407 A, for example).
However, the molds manufactured by these methods have a problem in that the mold swells because the calcium oxide generated when calcium carbonate is thermally decomposed at the time of burning of the mold absorbs moisture in the atmosphere and then turns into calcium hydroxide. When the mold swells, the surface roughness increases, the surface dimensional accuracy thereof decreases, and cracks occur in some cases. In addition, the moisture that has been absorbed in the mold is not dissociated from the mold unless the mold is heated to a temperature of not less than 580° C. Thus, unless the mold is heated to or above this temperature before the casting process, the moisture will be dissociated due to the heat of molten metal at the time of casting, thus causing porosities in some cases. Accordingly, the mold must be used in casting immediately after burning or must be stored with humidity sufficiently controlled. In addition, the mold cannot be stored for a long time since the mold inevitably absorbs moisture and swells even in a low humidity environment.
Meanwhile, there is known a technique to prevent a mold from self-collapsing due to the absorption of moisture in the atmosphere, by providing a backup coat layer containing calcium carbonate between an outermost surface coat layer and a face coat layer that is to be in contact with molten metal (refer to JP 2763970 B, for example). However, since both of the outermost surface coat layer and the face coat layer have air permeability, suppression of absorption of moisture of the mold is not sufficient.